= INTRAOSSEOUS LIPOMA

Frequency:<0.1% of primary bone tumors

Age: any (4^th–5^th decade); M÷F = 1.6÷1

May be associated with: hyperlipoproteinemia

• asymptomatic / localized bone pain (in up to 66%)

Milgram classification:

• Stage I viable large fat cells organized into lobules replacing bone marrow + encasing trabeculae
  • trabecular resorption + thin sclerotic rim
  • lucent lesion (–60 to 100 HU)
  • isointense to subcutaneous fat
• Stage II areas of partial fat necrosis accompanied by foamy macrophages + fibrosis associated with calcifications + reactive bone formation
  • regions of increased density (calcifications)
  • variable SI on T1WI + hyperintense on T2WI for fat necrosis
  • thick rim of reactive sclerosis
• Stage III almost complete involution of lipoma with cyst formation (= myxomatous degeneration) + thick radiodense border
  • expansile remodeling (DDx: osseous infarct)
  • regions of cyst formation (cystic degeneration)

Location in order of frequency:

• extremities (proximal femur >proximal tibia, fibula, humerus, radius), calcaneus, ilium, ribs, skull, mandible, maxilla, sacrum, coccyx, vertebrae; multiple locations = intraosseous lipomatosis

Site: metaphysis >diaphysis of long bones; epiphysis unusual

• expansile nonaggressive radiolucent lesion
• loculated / septated appearance (trabeculae)
• thin well-defined sclerotic border
• ± thinned cortex (NO cortical destruction)
• NO periosteal reaction
• may contain clumps of calcification centrally (= dystrophic calcification from fat necrosis)
  ◊VIRTUALLY DIAGNOSTIC in:
  • Proximal femur
    Site: intertrochanteric / subtrochanteric
    • marked ossification of margins of lesion
  • Calcaneus
    Site: in triangular region between major trabecular groups (on LAT projection)
    • calcified / ossified nidus
  • Ileum
    Site: adjacent to sacroiliac joint

◊Radiographic appearance similar to unicameral bone cyst (infarcted lipoma = unicameral bone cyst ?)

MR:

• similar to fat on all sequences (T1-hyperintense lesion)

Cx: rarely malignant transformation

DDx: fibrous dysplasia, simple bone cyst, posttraumatic cyst, giant cell tumor, desmoplastic fibroma, chondromyxoid fibroma, osteoblastoma